Recently, due to development of an image communication technology, a liquid crystal display device is being enlarged in a case of a floor type, and is being reduced in weight and thickness in a case of a portable terminal type. In the future, in order to realize a curved surface display of a large panel having a high realistic sensation and a take-up type portable terminal pursuing for portability and convenience, it is dispensable to make a substrate flexible as well as to reduce the thickness and weight thereof.
Conventionally, a glass substrate has been used in most cases as a substrate of a liquid crystal display device. A glass substrate has not only excellent transparency, solvent resistance, and gas barrier properties, but also high heat resistance sufficiently for withstanding a photoetching process, sputtering, and the like in the step of forming an alignment film and the step of forming an electrode in the production process of the liquid crystal device. However, when an attempt is made so as to reduce the thickness of a glass material constituting a glass substrate so that the glass substrate can be curved, shock resistance thereof becomes insufficient, which makes it difficult to handle the glass substrate.
The technology of using a resin film or a resin sheet, which has excellent shock resistance and is more light-weight and more excellent in flexibility than glass, in a base of a substrate for a display is being developed. However, a conventional resin substrate for a display device has heat resistance lower than that of an inorganic material such as glass. Therefore, particularly in the application to a thin film transistor (which may also be referred to as a TFT) substrate requiring a high-temperature process, a problem has been indicated that cracks are formed in electrodes to increase a resistance and cause disconnection.
When a resin having a high conjugate structure in which an aromatic ring or a hetero ring is incorporated into a main chain is used in order to enhance heat resistance, a substrate is colored necessarily, which makes it impossible to obtain sufficient display performance. As a resin substrate having transparency with heat resistance being considered, polycarbonate, polyarylate, polyethersulfon, or the like has been discussed. However, such a resin substrate still has insufficient heat expansibility (coefficient of linear expansion: dimensional stability) (specifically, the coefficient of linear expansion of a resin substrate is about 50 ppm° C.−1, which is about 10 times that of no-alkali glass for a display). Further, in the case of using a resin substrate, it is necessary to form a gas barrier layer on the surface thereof, which leads to increase in number of production steps, thereby causing problems such as the decrease in yield and the increase in cost.
On the other hand, a laminate substrate having an inorganic glass layer and a silicon oxide polymer layer has been proposed (for example, see Patent Document 1). According to Patent Document 1, the laminate substrate has excellent gas barrier properties, flexibility, heat resistance, and transparency. However, the laminate substrate in Patent Document 1 still has insufficient heat expansibility, and also has insufficient secondary processing characteristics and operability.
Patent Document 1: JP 2004-50565 A